Liquid crystal displays (LCDs) are becoming increasingly dominant in advanced visualization devices. LCDs offer favourable characteristics with respect to image quality (high luminance, high resolution, colour and grey scale capability), power consumption as well as dimensions and weight (flat panel displays). The use of commercial LCDs has become widespread, e.g. in automotive and telecommunication instruments, as well as in monitors of notebooks, desktop computers, television sets, etc. Today the need for LCDs in television applications is rapidly growing. Recently developed LCD modes possess high potentials in achieving fast response times, wide viewing angles and high luminance. Amongst other newly developed LCD modes, the MVA (multi-domain vertical alignment) mode appears to be the most promising for the use in modern television applications.
In the MVA mode the liquid crystal molecules are usually nearly vertically aligned with respect to the surface of the substrates. By using protrusions (or other alignment subdivisions) on the surface of the substrate, the liquid crystal molecules become locally pre-tilted within a single cell in more than one direction, leading to domains switchable in different directions. This multi-domain configuration exhibits very good display performance, with wide viewing angles of up to 160° in any direction, short response times (below 20 ms), high contrast ratios (up to 700:1) and high brightness.
However, by means of using protrusions only, it is difficult to clearly define the domain space within a single pixel. Therefore the MVA mode demands additional manufacturing steps to ensure shape effects as well as electrical field effects on both the upper and lower substrate; hence all in all leading to complex manufacturing procedures.
In order to by-pass this technical challenge, the availability of an alignment layer would be desirable, which directly leads to pre-defined alignment directions within each pixel domain and having well controllable off-axis angles with respect to the normal axis of the substrate.
Methods for the preparation of orientation layers for liquid crystal materials are well known to the skilled person. Customarily used uniaxially rubbed polymer orientation layers, such as for example polyimides, however, do have a series of disadvantages, like the formation and deposition of dust during the rubbing process and concomitant partial destruction of the thin film transistors.
Scratches due to brushing is another issue associated with this technique, which is particularly evident when the pixels are of the order of 10 microns or even lower, like e.g. in micro-display applications. Because of the strong optical magnification, which is required to visualize the displayed information, scratches easily become visible and are also the cause for the reduction of the contrast level. Furthermore, the rubbing process does not allow the production of structured layers.
The production procedure for obtaining orientation layers in which the direction of orientation is induced by irradiation with polarized light is not faced with the problems inherent to the rubbing process. With the irradiation technique it is furthermore also possible to create areas having different orientation and thus to structure the orientation layer as described for example in Jpn. J. Appl. Phys., 31 (1992), 215-564 (Schadt et al).
Using the linearly photo-polymerizable alignment (LPP) technique, the possibility of realizing a four-domain vertical aligned nematic (VAN) LCD was demonstrated some years ago (K. Schmitt, M. Schadt; Proceedings of EuroDisplay 99, 6-9 Sep., 1999). The four-domain VAN-LCD exhibits an excellent off-state angular brightness performance.
Apart from the current display performance requirements to be fulfilled in modern TV applications, the use of appropriate LPP materials is furthermore also guided by the necessity to achieve specific optical and electro-optical properties, e.g. with respect to the compatibility with the TFT (thin film transistors). Other important characteristics of the materials must also be taken into consideration, i.e. those crucial parameters directly related to and dependent on the molecular properties of the material. Primarily such characteristics are:                High voltage holding ratio (VHR), i.e. VHR of >90% (measured at 80° C.)        High stability of the induced pre-tilt angle against light and heat        Low alignment energy profile (short irradiation time and/or low irradiation energy)        
In the case of LCDs of thin-film transistor type a certain amount of charge is applied over the course of a very short period of time to the electrodes of a pixel and must not subsequently drain away by means of the resistance of the liquid crystal. The ability to hold that charge and thus to hold the voltage drop over the liquid crystal is quantified by what is known as the “voltage holding ratio” (VHR). It is the ratio of the RMS-voltage (root mean square voltage) at a pixel within one frame period and the initial value of the voltage applied.
Photo-reactive materials for orientation layers with improved voltage holding ratios (VHR) are described in WO-A-99/49360, JP-A-10-195296 corresponding to U.S. Pat. No. 6,066,696, JP-A-10-232400 corresponding to U.S. Pat. No. 6,027,772, WO-A-99/15576 and WO-A-99/51662. In WO-A-99/49360, JP-A-10-195296 and JP-A-10-232400 blends of polymeric compounds are described, containing photo-reactive polymers and polyimides.
In WO-A-99/15576 and WO-A-99/51662 polyimides having photo-reactive cinnamate groups incorporated in their side chains are described. WO-A-99/15576 for instance discloses photo-active polymers which contain as side-chain specific photo-cross-linkable groups and of which a typical monomer unit is 6-{2-methoxy-4-[(1E)-3-methoxy-3-oxoprop-1-enyl]phenoxy}hexyl 3,5-diaminobenzoate.